Overhead doors are used to occlude openings in structures such as warehouses, factories, and other commercial establishments. Doors are typically used at loading docks and are often subject to impacts from fork lifts, other loading devices, and freight. Such impacts often cause damage to an overhead door and sometimes to the building structure supporting the door. A variety of impact-resistant doors have been developed in response to this problem. For example, U.S. Pat. No. 5,584,333, issued to Torchetti et al., and U.S. Pat. No. 5,025,847, issued to Mueller, disclose assemblies designed to lessen the damage caused by an impact to a door.
While these and other prior-art devices have operated with some degree of success, they have several shortcomings. The impact resistant assemblies shown in U.S. Pat. No. 5,025,847, while operable to release from an associated track upon being exposed to force of a predetermined magnitude, are relatively complex in their mechanical arrangement. Complex door designs, of course, greatly increase the cost of manufacturing and maintaining an overhead door. Further, the door design taught by U.S. Pat. No. 5,584,333, while useful for reducing the damage to a flexible, bottom panel, is not appropriate for all applications, particularly those where a door with relatively rigid panels is desired.
Another shortcoming with prior-art doors is that they are constructed from relatively costly materials. For example, the door shown in U.S. Pat. No. 5,535,905, issued to Kellogg et al., includes extruded plastic tracks which are made from a relatively expensive low-friction material. In addition, the panels used in the door are made from numerous parts and relatively expensive non-metallic materials, including polycarbonate and fiberglass. While these doors are extremely sturdy, there are many instances where such a robust door is not required, and a lower-cost door would be more attractive to overhead door customers.
A further shortcoming with prior-art doors is that they do not provide sufficient thermal resistance (or insulation value). As should be apparent, the large openings in loading bays are a significant source of heat transfer (usually heat loss). Even when such openings are closed, heat transfer occurs through the doors themselves and through small openings around their edges and at their joints. A door with improved heat transfer resistance would reduce heat transfer and, therefore, the energy costs associated with maintaining a desired temperature within the building in which the door is located.
Therefore, it would be desirable to have an improved overhead door designed to release from its tracks when exposed to a force of a predetermined magnitude in order to limit or prevent damage to the door, its track, and surrounding structure. Further, it would be desirable if the door had a simple design with relatively few components and could be manufactured from relatively inexpensive materials so as to reduce the overall cost of the door. Further still, it could be desirable if the door had superior insulation characteristics.